Semiconductor laser device and method of manufacturing the semiconductor laser device

ABSTRACT

A semiconductor laser device includes a substrate, a buffer layer provided on an upper surface of the substrate and formed of InP, a laser element having a ridge structure formed above the buffer layer, and an epi intermediate layer formed of a compound semiconductor containing As and exposed to the outside.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device provided,for example, for high-speed optical response use and to a method ofmanufacturing the semiconductor laser device.

2. Background Art

As a semiconductor laser device realizing high-speed optical response,distributed feedback (DFB) laser devices which are for short-distancetransmission but which can be manufactured at a comparatively low costare mainly being used. There is a demand for achieving an improvement inyield (an improvement in the number of theoretically effective chipsfrom one wafer) of a semiconductor laser device by chip size shrinkageas well as a demand for achieving both realization of high-speedresponse by reducing the capacitance and an improvement inhigh-temperature characteristic (an improvement in heat dissipationeffect) by securing an epi volume about an emission point. From theviewpoint of improving the yield, it is desirable to form a process mesaby dry etching.

In DFB laser devices used for high-speed response in the presentcircumstances, the resonator length, for example, is set as short as 200microns or less in order to increase the relaxation-vibration frequency.Forming a process mesa by dry etching is indispensable to stabilizationof the characteristics of a DFB laser device having a short resonatorlength.

Japanese Patent Laid-Open No. 2003-051636 discloses a method ofmanufacturing a GaN-based semiconductor laser device.

Individual semiconductor laser devices separated from each other areobtained by cleaving a group of semiconductor laser devices in a waferstate. Cleaving is breaking the substrate at boundary portions between aplurality of semiconductor laser devices called dicing streets orseparating sections. There is a possibility of the substrate beingbroken at a portion other than the separating sections at the time ofcleaving. In particular, in a case where grooves called process mesagrooves are formed on opposite sides of ridge structure, there is aproblem that breakage can occur in portions reduced in thickness due tothe process mesa grooves.

SUMMARY OF THE INVENTION

In view of the above-described problem, an object of the presentinvention is to provide a semiconductor laser device and a semiconductorlaser device manufacturing method capable of dividing a substrate whilepreventing breakage of the substrate in portions reduced in thicknessdue to process mesa grooves.

The features and advantages of the present invention may be summarizedas follows.

According to one aspect of the present invention, a semiconductor laserdevice includes a substrate, a buffer layer provided on an upper surfaceof the substrate and formed of InP, a laser element having a ridgestructure formed above the buffer layer, and an epi intermediate layerformed of a compound semiconductor containing As and exposed to theoutside.

According to another aspect of the present invention, a method ofmanufacturing a semiconductor laser device includes a buffer layerforming step of forming a buffer layer on an upper surface of asubstrate, the buffer layer being formed of InP, an element forming stepof forming a plurality of laser elements on the buffer layer, each laserelement having a ridge structure, and a chip producing step of obtainingthe laser elements in chip form by dividing the substrate alongseparating sections which are regions including boundaries between theplurality of laser elements as viewed in plan, wherein the buffer layerforming step or the element forming step includes forming in theseparating sections an epi intermediate layer formed of a compoundsemiconductor containing As.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a semiconductor laser device in a waferstate;

FIG. 2 is a sectional view of a semiconductor laser device according toa comparative example;

FIG. 3 is a sectional view of the semiconductor laser device in a waferstate according to the second embodiment;

FIG. 4 is a sectional view of the semiconductor laser device in a waferstate according to the third embodiment;

FIG. 5 shows a semiconductor laser device; and

FIG. 6 is a sectional view of a semiconductor laser device in a waferstate according to a modified example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor laser device and a semiconductor laser devicemanufacturing method according to an embodiment of the present inventionwill be described with reference to the drawings. Components identicalor corresponding to each other are assigned the same reference numerals.Repeated descriptions of them are avoided in some cases.

First Embodiment

FIG. 1 is a sectional view of a semiconductor laser device in a waferstate. This semiconductor laser device in a wafer state is divided alongbroken lines in the future. The semiconductor laser device has asubstrate 12 formed of InP and a buffer layer 14 formed of InP providedon an upper surface of the substrate 12. The buffer layer 14 ispartitioned into an upper portion and a lower portion by an epiintermediate layer 16 formed in the buffer layer 14.

The material of the epi intermediate layer 16 is a compoundsemiconductor containing As. For example, the material of the epiintermediate layer 16 is InGaAsP, AlGaInAs, AlInAs or InGaAs. Thesecompound semiconductors can be easily lattice-matched to the bufferlayer 14 formed of InP. A plurality of laser elements having a ridgestructure are formed above the buffer layer 14.

Each laser element is, for example, a DFB laser element. The structureof the laser element will be described. A block layer 18 formed of InPis provided on the buffer layer 14. A cladding layer 20 formed of InP isprovided on the block layer 18. A contact layer 22 formed of InGaAs isprovided on the cladding layer 20. A feed metal 26 is formed on thecontact layer 22 with a passivation film 24 formed of an insulating filminterposed therebetween. The feed metal 26 is plated with Au plating 28.An active layer 42 is formed in the ridge structure 40. In the ridgestructure 40, an opening is formed in the passivation film 24 to bringthe contact layer 22 and the feed metal 26 into contact with each other.

At the boundaries between the plurality of laser elements, separatingsections 50 on which cleavage for separating the individual laserelements one from another are provided. The separating sections 50 areregions including the boundaries between the plurality of laser elementsas viewed in plan. The separating sections 50 extend generally parallelto the longitudinal direction of the ridge structures 40. The claddinglayer 20 is exposed in upper surfaces of the separating sections 50. Thesubstrate 12 is exposed in lower surfaces of the separating sections 50.

Journal of Material Science 23 (1988), 272-280 discloses the fact thatwhen the proportion of As in the composition of an InGaAs-based materialis increased, the Knoop strength (Knoop hardness) of the material isincreased and the material becomes easily breakable even under a reducedload. The document also discloses the fact that because, as a result ofan increase in proportion of As in the composition, the direction alongwhich the material can break easily is changed so that microcracks canoccur easily in the separating section, so that a state where thematerial is easily breakable even under a reduced load is established.Therefore, the epi intermediate layer 16 formed of a “compoundsemiconductor containing As” enables making the semiconductor laserdevice in the wafer state easily breakable in comparison with the casewhere the epi intermediate layer 16 does not exist.

If the layer thickness of the epi intermediate layer 16 is excessivelylarge, the probability of occurrence and growth of an oblique crack atthe time of cleavage is increased. Further, when the epi intermediatelayer is made thicker, the Knoop strength is correspondingly increasedand the probability of frequent occurrence of incomplete-separationfailure is increased. From the viewpoint of inhibiting these faults, itis desirable to set the layer thickness of the epi intermediate layer 16to a value equal to or smaller than 20 nm. From the viewpoint ofpreventing degradation in laser characteristics due to provision of theepi intermediate layer 16, it is preferable to make sufficiently thickthe buffer layer 14 above the epi intermediate layer 16 so that anactive layer light intensity distribution reaching the epi intermediatelayer 16 is reduced. For example, it is preferable to make the portionof the buffer layer 14 above the epi intermediate layer 16 sufficientlythick such that the light intensity distribution on the epi intermediatelayer 16 is 10% or less.

As described above, the light intensity distribution on the epiintermediate layer 16 is reduced and a thin film material which caneasily be lattice-matched to InP (e.g., InGaAsP, AlGaInAs, AlInAs orInGaAs) is used as epi intermediate layer 16, thus achieving good chipseparability without degrading the laser element characteristics.

A method of manufacturing the semiconductor laser device according tothe first embodiment of the present invention will subsequently bedescribed. First, the buffer layer 14 including the epi intermediatelayer 16 is formed on the upper surface of the substrate 12. This stepis referred to as a buffer layer forming step.

Thereafter, a plurality of laser elements having the ridge structure areformed on the buffer layer 14. This step is referred to as an elementforming step. In the element forming step, the active layer 42 is firstgrown to form the ridge (optical waveguide). Subsequently, the blocklayer 18, the cladding layer 20 and the contact layer 22 are formed inthis order. Subsequently, process mesa grooves 44 are formed by aprocess of dry etching.

Subsequently, passivation film 24, feed metal 26 and Au plating 28 areformed. Subsequently, the substrate 12 is ground at the lower surfaceside to be reduced in thickness. Subsequently, feed metal 30 and plating32 are formed.

Thereafter, a chip producing step for obtaining the plurality of laserelements in chip form is performed. In the chip producing step, each ofscribed lines is formed by applying a scribing blade to the lowersurface of one of the separating sections 50 in FIG. 1 (the lowersurface of the substrate 12) and sliding the scribing blade, and thesubstrate (wafer) is thereafter divided along the separating sections50. Since the wafer is made easily breakable by providing the epiintermediate layer 16, the substrate can be cleaved straight along thebroken lines shown in Fig. I. In the cross-section appearing as a resultof dividing the water into pieces (chips), the epi intermediate layer 16appears. That is, the epi intermediate layer 16 is exposed to theoutside.

FIG. 2 is a sectional view of a semiconductor laser device according toa comparative example. This semiconductor laser device has no epiintermediate layer. The substrate cannot be broken under a small loadunless an epi intermediate layer is provided. Therefore, there has beena possibility of application of a large load resulting in breakage ofthe wafer in portions reduced in thickness due to the process mesagrooves 44. In contrast, according to the invention set forth in Claim 1in the present application, the epi intermediate layer 16 is provided toenable the substrate to be broken under a small load, thus avoiding theproblem of breakage of portions reduced in thickness due to the processmesa grooves 44.

The semiconductor laser device and the semiconductor laser devicemanufacturing method according to the first embodiment of the presentinvention can be variously modified. For example, a semiconductor layerother than that shown in FIG. 1 may be provided. Semiconductor laserdevices and semiconductor laser device manufacturing methods accordingto embodiments described below have a number of commonalities with thedevice and method according to the first embodiment and will thereforebe described mainly with respect to points of difference from the firstembodiment.

Second Embodiment

FIG. 3 is a sectional view of the semiconductor laser device in a waferstate according to the second embodiment. Grooves 12 a are formed in thelower surface of the substrate 12 in separating sections 50. Each groove12 a may be stitch grooves (grooves intermittently formed) or acontinuous groove (a groove continuously formed). It is preferable thatthe grooves 12 a have a width which allows a scribing blade to enter thegroove, and which is about ⅙ of the chip width. The depth of the grooves12 a is not particularly specified if the grooves 12 a do not reach theepi intermediate layer 16. The depth of the grooves 12 a is, forexample, several microns to several tens of microns.

It is desirable to form the grooves 12 a by dry etching using anovolak-based negative resist mask or an insulating film hard maskcapable of maintaining a high resist selectivity and using a kind ofchlorine-based gas. The grooves 12 a are formed before the chipproducing step.

In the chip producing step, the substrate is cleaved along the grooves12 a. The provision of the grooves 12 a along with the epi intermediatelayer 16 enables individual semiconductor laser devices to be easilyseparated in the chip producing step. Thus, the substrate can be dividedwhile breakage of the substrate in portions reduced in thickness due tothe process mesa grooves 44 is prevented. Part of a peripheral portionof the substrate 12 of each separated individual semiconductor device issmaller in thickness than the portion surrounded by the peripheralportion by an amount corresponding to the formation of the groove 12 a.

Third Embodiment

FIG. 4 is a sectional view of the semiconductor laser device in a waferstate according to the third embodiment. An epi intermediate layer 60 isprovided only in the separating sections 50 by avoiding positions rightbelow the ridge structures 40. An epi intermediate layer is formed onthe entire area of the buffer layer 14 and unnecessary portions thereofare thereafter removed by etching, thus obtaining the epi intermediatelayer 60. Since the epi intermediate layer 60 is provided in eachseparating section 50, the substrate can easily be divided in the chipproducing step.

In the first and second embodiments, the epi intermediate layer 16 isformed through the entire semiconductor device, therefore it isnecessary to increase the thickness of the buffer layer 14 to such anextent that the epi intermediate layer 16 does not interfere with thedistribution of the intensity of light from the active layer. Formingthe thick buffer layer 14 requires a long epi growth time. In addition,in the case where the buffer layer 14 is thick, there is a possibilityof occurrence of cracking in the portions reduced in thickness due tothe process mesa grooves 44 before reaching the epi intermediate layer16 from the separating scribing point at the time of chip separation. Inthe third embodiment of the present invention, however, the epiintermediate layer 60 is provided only in the separating sections 50.Prevention of expansion of the light intensity distribution on the epiintermediate layer 60 is thereby enabled without increasing thethickness of the buffer layer 14.

In a case where an epi intermediate layer is provided only in theseparating sections 50, the epi intermediate layer can be provided in aplace other than the place in the buffer layer 14. That is, the epiintermediate layer can be provided in an intermediate portion of thecontact layer or in some other epi layer. FIG. 5 shows a semiconductorlaser device in which an epi intermediate layer 62 is provided in thecladding layer 20 in the separating sections 50.

Thus, an epi intermediate layer can be provided at any position otherthan the intermediate position in the buffer layer. That is, an epiintermediate layer formed of a compound semiconductor materialcontaining As can be formed in the separating sections 50 in the bufferlayer forming step or the element forming step. Restrictions on productsor the structure are thereby relaxed to ensure a higher degree of designflexibility. Also, because a certain degree of flexibility in terms ofscribing load at the time of chip separation is obtained, the structureof the obtained semiconductor laser device can be stabilized. That is,the stability of chip separation can be improved.

FIG. 6 is a sectional view of a semiconductor laser device in a waferstate according to a modified example. Epi intermediate layers 60 and 62are formed only in the separating sections 50, and grooves 12 a areformed in the lower surface of the substrate 12. As a result, a furtherimprovement in separation stability can be achieved. Furthermore, thefeatures of each of the above-described embodiments may be combined andused as needed.

According to the present invention, an epi intermediate layer easy tobreak is provided at the boundaries between the semiconductor laserdevices, thereby enabling dividing _(t)he substrate while preventingbreakage of the substrate in portions reduced in thickness due toprocess mesa grooves.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A semiconductor laser device comprising: asubstrate; a buffer layer provided on an upper surface of the substrateand formed of InP; a laser element having a ridge structure formed abovethe buffer layer; and an epi intermediate layer formed of a compoundsemiconductor containing As and exposed to the outside.
 2. Thesemiconductor laser device according to claim 1, wherein part of aperipheral portion of the substrate is smaller in thickness than theportion surrounded by the peripheral portion.
 3. The semiconductor laserdevice according to claim 1, wherein the epi intermediate layer isprovided by avoiding a position right below the ridge structure.
 4. Thesemiconductor laser device according to claim 1, wherein the layerthickness of the epi intermediate layer is equal to or smaller than 200nm.
 5. The semiconductor laser device according to claim 1, wherein theepi intermediate layer is formed as an intermediate layer in the bufferlayer, and the portion of the buffer layer above the epi intermediatelayer is formed with a thickness such that a light intensitydistribution from the epi intermediate layer is 10% or less.
 6. A methodof manufacturing a semiconductor laser device, comprising: a bufferlayer forming step of forming a buffer layer on an upper surface of asubstrate, the buffer layer being formed of InP; an element forming stepof forming a plurality of laser elements on the buffer layer, each laserelement having a ridge structure; and a chip producing step of obtainingthe laser elements in chip form by dividing the substrate alongseparating sections which are regions including boundaries between theplurality of laser elements as viewed in plan, wherein the buffer layerforming step or the element forming step includes forming in theseparating sections an epi intermediate layer formed of a compoundsemiconductor containing As.
 7. The method of manufacturing asemiconductor laser device according to claim 6, further comprising astep of forming grooves in a lower surface of the substrate in theseparating sections before the chip producing step, wherein, in the chipproducing step, the substrate is cleaved along the grooves.